This invention broadly relates to an article comprising: a silicon-containing substrate; an overlaying silicide-containing bond coat layer; and an overlaying environmental barrier coating (EBC) that includes a corrosion resistant outer layer comprising a corrosion resistant metal silicate. This invention further broadly relates to processes for forming the corrosion resistant outer layer of the EBC over the silicide-containing bond coat layer.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for gas turbine components used throughout gas turbine engines, and especially the higher temperature sections, alternative lighter weight substrate materials have been proposed and sought.
Ceramic materials containing silicon, such as those comprising silicon carbide (SiC) as a matrix material and/or as a reinforcing material (e.g., as fibers) are currently being used as substrate materials for higher temperature applications, such as gas turbine engines, heat exchangers, internal combustion engines, etc. These silicon-containing matrix/reinforcing materials are commonly referred to as ceramic matrix composites (CMCs). These silicon-containing materials used as matrix materials and/or as reinforcing materials can decrease the weight yet maintain the strength and durability of turbine components comprising such substrates, and are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as turbine components comprising airfoils (e.g., compressors, turbines, vanes, etc.), combustors, and other turbine components for which reduced weight is desirable.
As operating temperatures increase, the high temperature durability of such CMC materials must also correspondingly increase. In many applications, a protective coating is beneficial or required for such silicon-containing substrates. Such coatings should provide environmental protection by inhibiting the major mechanism for degradation of silicon-containing materials in a corrosive water-containing environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, a necessary requirement of an environmental barrier coating (EBC) system for a silicon-containing substrate is stability in high temperature environments containing water vapors. Other important properties for these coating systems can include a coefficient of thermal expansion (CTE) compatible with the silicon-containing substrate, low permeability for oxidants, low thermal conductivity, and chemical compatibility with the silicon-containing substrate and overlaying silica scale formed typically by oxidation.
Various single-layer and multilayer EBC systems have been investigated, but each has exhibited shortcomings relating to environmental protection and compatibility with silicon-containing substrates. For example, EBC systems have been suggested for protecting silicon-containing CMC substrates from oxidation at high temperatures and degradation in the presence of aqueous environments (e.g., steam). These steam-resistant EBC systems include those comprising mullites (3Al2O3.2SiO2) disclosed in, for example, commonly-assigned U.S. Pat. No. 6,129,954 (Spitsberg et al.), issued Oct. 10, 2000, and U.S. Pat. No. 5,869,146 (McCluskey et al.), issued Feb. 9, 1999. Other steam-resistant EBC systems comprising barium strontium aluminosilicate (BSAS), with or without mullite, and with or without additional thermal barrier coatings are disclosed in, for example, commonly-assigned U.S. Pat. No. 5,985,470 (Spitsberg et al.), issued Nov. 16, 1999; U.S. Pat. No. 6,444,335 (Wang et al.), issued Sep. 3, 2002; U.S. Pat. No. 6,607,852 (Spitsberg et al.), issued Aug. 19, 2003; and U.S. Pat. No. 6,410,148 (Eaton et al.), issued Jun. 25, 2002.
One version of these steam-resistant EBCs comprise an essentially three-layer system of: (1) a silicon bond coat layer adjacent the silicon-containing substrate; (2) a combination mullite-BSAS (e.g., 80% mullite-20% BSAS) transition layer overlaying and adjacent the bond coat layer; and (3) an outer barrier layer comprising BSAS. See, e.g., commonly assigned U.S. Pat. No. 6,410,148 (Eaton et al.), issued Jun. 25, 2002. The silicon bond coat layer provides good adhesion to the silicon-containing substrate (e.g., a SiC/SiC CMC substrate) and can also function as a sacrificial oxidation layer. The mullite-BSAS transition layer prevents rapid reaction between the outer barrier layer comprising BSAS and the underlying silica scale that typically forms on the silicon bond coat layer. The outer barrier layer comprising BSAS is relatively resistant to steam and other high temperature aqueous environments.
These steam-resistant EBCs comprising BSAS are typically deposited on the silicon-containing CMC substrates by thermal spray techniques such as plasma spraying. Plasma spraying tends to form relatively thick coatings or layers that may not be suitable for certain applications. In addition, these steam-resistant EBCs comprising BSAS may also not be sufficiently resistant to other forms of environmental attack.
These steam-resistant three-layer EBC systems were originally developed for gas turbine component applications where the EBC surface temperature of the silicon-containing CMC substrate did not exceed about 2200° F. (1204° C.). Future gas turbine component applications are expected to increase the EBC surface temperature of the silicon-containing CMC substrate well above about 2200° F. (1204° C.).
Some thermal insulation from these expected higher surface temperatures can be addressed by including one or more thermal barrier coating (TBC) layers on top of the three-layer EBC system. See commonly assigned U.S. Pat. No. 6,444,335 (Wang et al.), issued Sep. 3, 2002 (T/EBC system that comprises a thermal insulating YSZ top coat layer overlying an intermediate layer containing YSZ and BSAS, mullite and/or alumina that overlies a mullite-containing layer that can be adhered to the silicon-containing substrate by an optional silicon layer.) Even with these additional TBC layers, the silicon-containing CMC substrate, as well as the silicon bond coat layer, is still expected to experience effective temperatures well above about 2200° F. (1204° C.).
Accordingly, it would be desirable to be able to provide an environmental barrier coating (EBC) for silicon-containing (e.g., CMC) substrates that can be formed to: (1) provide coating thicknesses that are thinner than those provided by thermal spray techniques such as plasma spray; and/or (2) are resistant to environmental attack by other corrosive agents besides high temperature aqueous environments (e.g., steam). It would further be desirable to be able to provide a bond coat layer that can adhere such an EBC to the silicon-containing (e.g., CMC) substrate, even when experiencing effective interface surface temperatures between the EBC and the substrate that are well above about 2200° F. (1204° C.).